In the cutting of metals, e.g. steel, it is a common practice to make use of an autogenous process in which the metal, raised to an appropriate temperature, reacts autogenously with oxygen supplied by the torch and is thus burnt away to form a kerf or cut along the line described by the torch.
In its simplest form, a device for this purpose may consist of a nozzle directing a jet of oxygen against a workpiece which has been heated to the necessary temperature.
In practical applications, such torches have been modified to provide the means for heating the workpiece at the location to be subjected to the oxygen jet and to shield the oxygen jet from detrimental influences, e.g. the entrainment of air which has the effect of diluting the cutting oxygen and, by reducing its purity, decreasing the cutting efficiency and/or precluding continued cutting operations and/or causing irregularities in the curve or cut which is produced.
It has been proposed, therefore, as described for example in Swiss Pat. No. 442 939 to provide a torch whose nozzle is formed with a central passage through which the jet of cutting oxygen is directed at the workpiece, at least one passage surrounding this central passage for discharging around the cutting jet a curtain of oxygen serving to exclude air from the cutting jet.
In the system of this Swiss patent, the workpiece is heated by a preheating flame trained upon the workpiece, the preheating flame being directed at an angle to the cutting gas stream or concentrically surrounding same.
The use of an oxygen-containing curtain surrounding the high-purity cutting oxygen stream has been found to markedly improve the cutting efficiency of autogenous cutting burners or torches since the central cutting jet is maintained free from impurities arising from the surrounding preheating gas or from the air in the region of the cutting operation. The high oxygen content or purity of the cutting jet is thus ensured.
Apart from the cutting efficiency, however, there are several parameters of the operation of a cutting torch which must be taken into consideration for satisfactory cutting for a given sheet thickness and workpiece composition.
For example, it is important to maintain the highest possible cutting efficiency and cutting rate while maintaining the maximum cutting precision, i.e. tracking of the kerf along a predetermined path. The cut should be as regular or even as possible and the chamfering depth should be minimized as should the melting of the surface of the workpiece adjacent the kerf, i.e. at the upper edge of the kerf.
By comparison with cutting operations without any oxygen curtain, the oxygen-curtained torch significantly reduces, for otherwise identical conditions, the unevenness of the cut, chamfering of the workpiece and the cleanness of the cut vis-a-vis damage to the workpiece surfaces. However, precision is not as advantageously affected since a narrow kerf along a predetermined path cannot be completely ensured. The problem is most noticeable when the torch is used to cut along curved lines.
To reduce chamfering and increase the precision of the cut it has been proposed to increase the pressure of the oxygen jet. In practice however, this has been found to invariably give rise to a significant increase in the unevenness of the cut, either by the formation of bulging flanks along the cut or, even more commonly, strongly divergent flanks. With an increase in the oxygen pressure at the inlet to the cutting nozzle, there is thus a singificant increase in the width of the cut below the groove formed at the top and the upper surface of the workpiece. The widened kerf is naturally disadvantageous when the cutting operation is to produce products of predetermined shape and size.